I've done some work with the pRSET vectors and had decent success.
One of my observations was that in small cultures (3-5 ml) I couldn't detect
the protein. But when a large culture is grown (500 ml - 1 litre) I saw an
obvious overexpression.
OD values seem to be specific to individual fusion proteins - try various
conditions (induce the cells in increasing cell densities : 0.4, 0.6, 0.8
and 1.0 OD600 values).
Try 0.1mM, 0.5mM IPTG concentrations to induce the protein.
(These can only be done after following another suggestion to your query -
sequence the clones to verify if your protein is in frame with the tag.
Some clones get induced better than others. Once I identified cell clones
expressing well I made glycerol stocks and have been using those cells
eversince).
Hope this helps.
Vayuputra